Studies of the administration of testosterone and estradiol to normal men at physiological dosage demonstrate that estradiol, which is produced primarily by peripheral conversion of testosterone, accounts for the majority of the suppressive effects of testosterone on LH and FSH secretion. Blockade of estradiol action, by use of the antiestrogen, clomiphene, completely reverses the suppressive effects of testosterone on plasma gonadotropin concentrations. The data emphasize the important role of peripheral aromatization of testosterone in its physiological interaction in the hypothalamus and pituitary.